A major cause of failure in polyphase electric motors is the breakdown of the insulation which protects the motor windings. Such degradation can ruin an expensive motor. Moreover, in refrigeration systems which employ hermetically sealed motors, insulation failure can cause arcing which burns the plastic insulation and contaminates the refrigerant piping. This may necessitate expensive and time-consuming repairs to the system.
Traditionally, motors have been protected from sudden failure by devices such as circuit breakers and temperature switches which respond to excessive current or temperature by shutting down the motor before it fails. However, neither of these devices provides any indication of the cause of the problem or any warning of impending motor shutdown.
A number of devices employ an AC signal to monitor insulation faults. However, these systems typically require complex electronics and coupling devices. Certain other systems utilize a simpler DC detection circuit. However, such systems typically can be used only when the motor is not operating. And in a number of circumstances shutting down a motor can be very undesirable or even wholly unacceptable, such as when it is being employed in a large refrigeration system, such as in a frozen food warehouse, a hospital's water system, a cooling system for a missile or nuclear reactor, or in similar applications where continuous motor operation is essential. Another technique, utilized in refrigerant compressors, involves periodically testing the crankcase oil for acidity which can attack the insulation. Again, this technique requires that the compressor be shut down and also requires extensive operator intervention to open and close valves, take oil samples and check for leaks.
One device does employ DC detection to monitor insulation faults while the motor remains operational. However, it operates only with an ungrounded electrical power source and inasmuch as most power sources are grounded, this device has limited application. It also shuts down the motor which as indicated above may be very undesirable.